Re: [PATCH v7 07/12] dma-mapping: introduce dma_has_iommu()

[Dan Williams <dan.j.williams@intel.com>]

On Mon, Oct 9, 2017 at 11:58 AM, Jason Gunthorpe
<jgunthorpe@obsidianresearch.com> wrote:
> On Fri, Oct 06, 2017 at 03:35:54PM -0700, Dan Williams wrote:
>> otherwise be quiesced. The need for this knowledge is driven by a need
>> to make RDMA transfers to DAX mappings safe. If the DAX file's block map
>> changes we need to be to reliably stop accesses to blocks that have been
>> freed or re-assigned to a new file.
>
> If RDMA is driving this need, why not invalidate backing RDMA MRs
> instead of requiring a IOMMU to do it? RDMA MR are finer grained and
> do not suffer from the re-use problem David W. brought up with IOVAs..

Sounds promising. All I want in the end is to be sure that the kernel
is enabled to stop any in-flight RDMA at will without asking
userspace. Does this require per-RDMA driver opt-in or is there a
common call that can be made?

Outside of that the re-use problem is already solved by just unmapping
(iommu_unmap()) the IOVA, but keeping it allocated until the eventual
dma_unmap_sg() at memory un-registration time frees it.

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